1. Field of the Invention
The present invention relates to a method of molding a polyurethane product which has variable physical properties, such as, different colors, hardness, etc., between the surface and inner layers. The invention also relates to a molding apparatus.
2. Description of Related Art
The conventional RIM technology for molding polyurethane parts includes applying a mold release agent to the surface of the mold cavity. If necessary, an insert is also set in the cavity of the mold. A RIM polyurethane material is, then injected into the cavity so that the respective components will react to cure. Then, the mold is opened and the molded part is removed.
Among these steps, the application of the mold release agent accounts for a large portion of the molding cycle and hence, it is an impediment to reducing the cycle time. Furthermore, the use of a single type of RIM polyurethane material makes it difficult to provide different properties in the surface and the interior layers of the molded part. Presently, the only way known is providing different blow ratios. It has been impossible to achieve various characteristics, such as a feeling of touch, and endurance without defects.
Although polyurethane molding by RIM is sometimes done after pigment-containing polyurethane materials (colorants) are applied to the surface of the mold cavity, such an application step is time-consuming.